I've just destroyed my dev instance of Roundcube 1.7 RC 2 by installing the security update 1.7 RC 3 as it is using functions like array_last(array $array) which are only available in PHP 8.5 and higher. Please include this into the Changelog as to me this seems to be undocumented.
1.7 supports PHP >=8.1 <=8.5. What method did you use to perform your upgrade? It sounds like you are missing the symfony/polyfill-php85 library. However that has been needed since the first release candidate so it's odd that you are only having problems now.